The importance of fluid loss control in primary oil well cementing has been recognized for many years. Adequate filtrate loss control is a major concern when cementing across any producing zone. As the cement slurry is being displaced across a permeable zone, it tends to lose fluid to that zone and, as a result, a filter cake tends to build up from the dehydrated cement particles. If the displacement process continues, the result can be a prematurely dehydrated cement slurry. The lost fluid from the cement slurry can also contaminate the surrounding formations.
The filtrate loss which results when a cement slurry does not contain fluid loss additives affects the thickening time of the slurry and the setting time with the result usually being premature setting of the cement slurry and a decrease in the compressive strength of the set cement column. Premature dehydration of a cement slurry can also cause flow of formation fluids into the well annulus, as well as between producing zones. When cement slurry filtrate loss is excessive, the filter cake can build radially inward from the permeable formation to the pipe, causing the annulus to be blocked off or bridged. Further displacement of the slurry is then prevented and the hydrostatic pressure originally trapped below the bridged area is reduced. If the slurry pressure is reduced to less than the pressure in any of the zones below the bridged area, intrusion of gas and fluid into the well annulus and between zones can occur.
Fluid loss additives are, therefore, used in oil well cement slurries to prevent problems associated with filtrate loss of the cement slurry, including: (1) excessive filter cake buildup, (2) prematurely dehydrated cement slurries, (3) filtrate damage to the surrounding producing formations, (4) changes in the thickening time of the cement slurry and the subsequent possibility of premature setting of the slurry, (5) changes in the setting time of the cement column and a resulting decrease in the compressive strength properties of the set cement, and (6) bridging in the annulus which can lead to flow of formation fluids into the annulus and between zones.
Polyamines, including polyethylenimines, are known to be highly effective cement fluid loss control additives when used with companion materials such as lignosulfonates, formaldehyde condensed napththalene sulfonates or other sulfonated polymers. A particular advantage of such systems is their ability to obtain very good fluid loss control at relatively low cement consistencies. Because of the low cement consistencies associated with polyethylenimine systems, settling and free-water can sometimes be a problem. Settling of the cement slurry can lead to a poor cementing job and increase the chance of communication between zones in the well. Salts such as calcium chloride have been added to fresh water cement slurries in the past in an attempt to control settling and free water problems where lignin based companion materials are used with polyamine fluid loss additives. The use of salts does not always eliminate all settling problems, adds to the cost of the slurry, and can damage salt sensitive formations.
There exists a need, therefore, for a polyamine fluid loss additive for oil well cements which is effective without producing settling and free water problems.
There exists a need for such a fluid loss additive which is economical to use and which does not greatly increase the cost of the cement slurry.